The present invention relates to a surface wave for suppressing reflections of terminal transducer fingers transducer.
In order to suppress undesired reflections of the acoustic wave in the transducer and on its transducer fingers, it has been known for a long time for such (transmitting/receiving) transducers to be constructed with interdigitally arranged transducer fingers, especially as split finger transducers. In comparison with a simple interdigital transducer with adjacent transducer fingers having alternating polarity, such transducers have in each case two (if required, also in each case three) adjacent fingers which have the same polarity, that is to say they are electrically connected to one and the same busbar. In a normal, regular design of such a split finger transducer, there follow alternating with one another two (or three) transducer fingers connected to the one busbar and two (or three) transducer fingers connected to the other busbar. The center distances between successive transducer fingers (irrespective of which polarity, that is to say including the fingers within a "split finger" and apart from points at which transducer fingers are omitted) are a quarter (or a sixth) of the acoustic wavelength of the surface wave to which the relevant transducer is tuned, to be precise the wavelength of the acoustic wave in the substrate material of the transducer. The center distances of adjacent groups of two (or three) adjacent fingers of the same polarity, that is to say the center distances between alternating split fingers, is once again equal to half the wavelength, as in the case of simple transducers.
A further transducer type with suppression of internal reflections of the ultrasound waves is the third-lambda finger arrangement. This is likewise an interdigital transducer in which, as in the case of the split finger transducer, the one finger comb (with the busbar) has doubled transducer fingers and there is in each case a single finger of the finger comb of the other busbar between adjacent groups of such double transducer fingers.
For the internal regions of such a split finger transducer or third-lambda transducer, the neutralization or suppression of internal reflections in the transducer is virtually complete with correspondingly precise dimensioning. However, this unfortunately does not apply to the terminal split finger groups and finger groups. In order to achieve suppression of reflections for these as well, it is known (European reference EP-A-0,088,400) for the relevant transducers to be provided with terminal transducer fingers which are dimensioned such that their finger length is reduced in steps, in such a manner that terminal transducer fingers interact in an interfering manner with one another with respect to the reflections. A disadvantage of such an (additional) measure is, however, that such an end of stepped construction of a transducer causes distortions in the phase front of the acoustic wave of the transducer. In addition, not all the fingers of a transducer which is weighted in an overlapping manner, as is often the case, are affected completely by the acoustic wave which is received by the transducer which is operated as the receiving transducer. The efficiency of such a measure for suppressing undesired reflections occurring at the end of a transducer is also only limited.
Another method of suppressing reflections occurring on terminal fingers of an interdigital transducer is that which is described in German reference DE-B-2,839,851. Weighted fingers in the region of the transducer end are directed with essentially longitudinal components of these fingers at an oblique angle with respect to the major axis of the wave propagation, so that these fingers reflect ultrasound waves which impinge on them away in an oblique direction, by means of these obliquely positioned components of their finger lengths, that is to say said fingers do not intrinsically reflect back.
A further method (Trans. IECE Jap. Vol 64-C (1981) pages 437-438) is to design some of the terminal fingers to be shortened with respect to the mutual distance between busbars which are opposite one another, and to replace the "missing" finger length by a floating finger in each case. However, floating potentials are highly problematic in many applications.
Finally, reference should also be made to a method of suppressing reflections of terminal fingers which consists of the outermost finger of the two fingers of the terminal (consisting of these two fingers) split finger being designed to be directed obliquely with respect to the major axis direction of wave propagation. With this measure, it must be accepted that interference occurs in the phase front of the acoustic wave of the useful signal.
Reference should also be made here to U.S. Pat. No. 4,162,465 which is apparently similar to the invention which is also described in the following text. This document describes a split finger transducer in which, in contrast to split finger transducers which were known at that time and in which wave reflections are suppressed in an artificial manner at the fingers, such mechanical reflections are caused deliberately. The purpose of this measure is to compensate for, that is to say to eliminate (reflected) waves which are reflected on the electrical load by means of such deliberately caused waves. To this end, according to the teaching of this document, it is necessary, to be precise within in each case one of the two transducer combs of the interdigital transducer, to provide fingers with a finger width which deviates from the other fingers, and a deviating finger separation, one or more such fingers of the one comb being adjacent to one or more fingers of the other comb. The mechanical (MEL) wave, which compensates for the wave which is reflected electrically (regenerated) by the other load, is produced in the interior of the thus modified transducer. The dimensioning, which for this purpose is dependent on the impedance of the load, of these deviating fingers is specified. This has nothing to do with the suppression of reflections of terminal fingers of a transducer and the fingers, dimensioned according to U.S. Pat. No. 4,162,465, of the transducer described there are also not effective for this purpose.